I read somewhere that the F ratio when describing telescopes should not be compared to the F ratio when referring to camera lens
Telescope Attributes - Telescopes are designed to gather light and bring it to focus so that the image can be examined in detail with an eyepiece, or recorded on film or with a digital camera. Telescopes reveal fainter objects than can be seen with the eye because they gather more photons than the eye can gather. Smaller details can be seen because they also magnify objects. The nomenclature used to describe telescopes and camera lenses can sometimes be confusing. Telescopes are usually talked about in terms of aperture, while camera lenses are usually talked about in terms of focal length. Most people will say they have an 8 inch telescope (meaning aperture), but they will also say them have a 300 millimeter camera lens (meaning 300mm of focal length). No wonder it's confusing! But we can easily sort this out.
Telescopes and camera lenses have three main numerical attributes that we are concerned with in describing them:
- Aperture - The aperture is the size of opening in the telescope through which the lens or mirror gathers light. It is the most important attribute of a telescope because light gathering is what telescopes are all about. In astrophotography, the larger the aperture, the more photons can be collected. Aperture, however, is not the only criteria for judging a telescope. Optical quality is just as important. You can have a gigantic aperture and if the optical quality of the telescope is not good, the light won't be very well focused, and the images produced won't be very good. Aperture is the main determinant in how faint of a star you can see with a telescope. The down side to aperture is that as the size of the aperture goes up, so does the cost and complexity of making the optical system, as well as the weight and size. Bigger apertures also usually mean more focal length, and this makes mounting them, carrying them around and using them more difficult, especially for astrophotography. Aperture is measured in inches or millimeters (mm). There are 25.4 mm in an inch, so a 4-inch aperture telescope has an aperture of 101.6 mm.
- Focal Length - The focal length of a telescope is the distance from the objective lens or mirror at which the light comes to focus. The longer the focal length, the larger the image is that forms at the focal plane, and the higher the magnification of the telescope. Increased magnification with longer focal lengths is a good thing for small objects like planets and double stars, but undesirable things also get magnified, like poor atmospheric seeing, and imperfections in the telescopes drive and wobble in the mounting.
Focal length is also measured in inches or millimeters. Camera lenses usually give the focal length in millimeters. A simple lens with a focal length of 300 mm will form the image 300 mm behind the lens. Some telescopes have a secondary mirror that bends the light path, sometimes even folding it back on itself, making the physical length of the instrument much shorter than the focal length would imply.
- Focal Ratio - The focal ratio is the relationship between the aperture and focal length. The focal ratio is defined as the focal length divided by the aperture. For example, a refractor with a focal length of 800mm and an aperture of 100mm has a focal ratio of 800/100 = 8 or f/8. The focal ration gives the relative "speed" of the optical system. This is important for recording extended objects such as nebulae and galaxies. A faster focal ratio will record an image faster (with a shorter exposure).
Focal ratio is also known as the f/ratio, and is described by the f/number.
For example, a 4 inch refractor has an aperture of about 100 millimeters. If the focal length of this scope is 500 millimeters, then we can determine the f/number by dividing the focal length by the aperture, which in this case is 500 / 100 = 5. So we say this scope has an f/ratio, or focal ratio, or f/number of f/5.
F/5 is a mid-range f/number. Mid-range f/ratios are usually about f/5 to f/8. "Fast" f/ratios are usually considered about f/4 or lower, such as f/2.8 or f/2. You won't usually find f/ratios this fast in a telescope, but you definitely will in camera lenses. Slow f/ratios are anything bigger than f/9 or so.
F/ratios are also known as f/stops in photography. Each f/stop is equal to a doubling or halving of the amount of light. For example, an f/ratio of f/4 lets in twice the amount of light as an f/ratio of f/5.6 and requires half the exposure.
The full f/stop series, in one stop increments is:
f/1 f/1.4 f/2 f/2.8 f/4 f/5.6 f/8 f/11 f/16 f/22 f/32 f/64
These numbers continue on each end of the scale, but these are the practical working range of f/stops.
Each of these f/stops is equal to a one-stop difference in light getting through. So every time you change the f/stop by one full increment, you also have to change the shutter speed, or exposure time, by doubling or halving the exposure to compensate.
For example, at the same ISO (Film speed or digital camera sensitivity), a 1 second exposure at f/5.6 would equal a 2 second exposure at f/8, or a 1/2 second exposure at f/4. All would be equivalent.
Here is a list of equivalent exposures, all allowing the same amount of light to reach the sensor:
f/1 f/1.4 f/2 f/2.8 f/4 f/5.6 f/8 f/11 f/16 f/22 f/32 f/45 f/64 1/1024
sec 1/512
sec 1/256
sec 1/128
sec 1/64
sec 1/32
sec 1/16
sec 1/8
sec 1/4
sec 1/2
sec 1
sec 2
sec 4
sec
For simplicity in the short exposures, the higher shutter speeds are rounded off, such as 1/32nd sec is rounded to 1/30th sec, 1/64th to 1/60th, 1/128th to 1/125, 1/256th to 1/250th, 1/512th to 1/500th and 1/1024th to 1/1000th. The differences are so small as to be inconsequential.
If you take a camera lens with a fixed focal length, and stop down the lens, and look at the lens from the front, into the camera, you will see the size of the hole made by the diaphragm blades gets smaller as the f/number gets bigger. f/32 is a very small hole compared to f/2.8. f/32 is a "slow" aperture because the small hole does not let a lot of light get in over the same time exposure as a larger hole. It's "slow" because it requires a longer exposure.
Long focal length instruments with slow focal ratios will work well for bright objects like the Sun, Moon and planets. You can get by with scopes with high f/numbers because the exposures will still be reasonably short. Long focal length instruments also have small fields of view.
Short focal length instruments have wider fields of view and usually have faster focal ratios and can record faint extended objects faster.